As someone who spent over a decade catering luxury events, let me add some back of house info:
These events are almost always open bar. They're not trying to make their money back on alcohol. They want you to drink and eat and donate generously.
If there are cocktails, there will be at most two on offer, pre-made in large tubs. You cannot order a different version, it is what it is.
There are two types of events: cocktail style or seated. The first includes roaming hors d'oeuvres or a fancy buffet with tiny plates called a grazing station. For a long night, the roaming food will get a little bigger throughout the evening and have a 'main' at some point based around a protein.
A seated event will usually be more structured and may include multiple courses. Silver service is not in vogue anymore. You are likely to get either alternating meals brought to you like at a wedding, or served banquet style. A good caterer can get a plate to everyone in a 300 person event in about three minutes.
Drunk people are the same no matter how expensive their suits. They still laugh too loud, spill their drinks and slip on the dance floor. They are usually less embarrassed about doing coke in the bathrooms.
A full scale event that starts at 6pm will have staff arriving at noon to begin setup. Earlier if there's a light show or pyrotechnics. Typically venues don't just have 30 tables and three hundred chairs lying around, let alone table cloths, chair covers, etc. It's all rented and brought in on the day. Bands and DJs will be running audio tests in the background throughout.
Most heritage buildings that host these things, like museums and manor houses, aren't really designed for them. They might put down mats so you're not walking in stilettos over two hundred year old wooden floors, the kitchens are weirdly far away, and there are not enough taps. There is never anywhere for staff to sit, so if you open the wrong door you might find half a dozen waiters sitting on upturned milk crates in a room full of million dollar paintings, eating the left over bread.
Really old buildings don't have enough bathrooms, which means the staff will be sharing with the guests.
Clean up starts the second the event ends, if not sooner. Unattended glasses will start to disappear first, then table decorations. When the timer ticks over, the lights come back on and exhausted staff strip the tables, pack up dirty glasses and unopened wine bottles and have to Tetris it all into the back of a van. The venue is booked for that day only, so everything has to be gone before anyone can go home. A large event that finishes at midnight might take until 3am to be cleared away.
These are very long and physically demanding nights for anyone working them. The staff all get to know each other, and will absolutely notice someone trying to sneak in wearing a borrowed uniform. They are not being paid enough to care.
